During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom,
and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. The yarns of the fabric that run along the direction of paper machine operation are referred to as the machine direction (MD) yarns; and the yarns that cross the MD yarns are referred to as the cross machine direction (CD) yarns. It should further be appreciated that paper manufacture is a continuous process, which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Traditional press sections include a series of nips formed by pairs of adjacent cylindrical press rolls. Recently, the use of long press nips has been found to be advantageous over the use of nips formed by pairs of adjacent rolls. The longer the web can be subjected to pressure in the nip, the more water can be removed there, and, consequently, the less will remain to be removed through evaporation in the dryer section.
In long nip presses of the shoe type variety, the nip is formed between a cylindrical press roll and an arcuate pressure shoe. The latter has a cylindrically concave surface having a radius of curvature close to that of the cylindrical press roll. When roll and shoe are brought into close physical proximity, a nip is formed which can be five to ten times longer in the machine direction than one formed between two press rolls. This increases the so-called dwell time of the fibrous web in the long nip while maintaining the same level of pressure per square inch pressing force used in a two-roll press. The result of this new long nip technology has been a dramatic increase in dewatering of the fibrous web in the long nip when compared to conventional nips on paper machines.
A long nip press of the shoe type typically needs a special belt. This belt is designed to protect the press fabric supporting, carrying, and dewatering the fibrous web from the accelerated wear that would result from direct, sliding contact over the stationary pressure shoe. Such a belt is made, for example, with a smooth impervious surface that rides, or slides over the stationary shoe on a lubricating film of oil. The belt moves through the nip at roughly the same speed as the press fabric, thereby subjecting the press fabric to minimal amounts of rubbing against stationary components.
In addition to being useful in a long nip press, the present invention also relates to process belts used in other papermaking and paper-processing applications, such as calendering used to smooth paper surfaces taking advantage of the longer period that the paper web is under a pressure load. Furthermore, other belts used for transferring paper webs in the papermaking process also are subjected to environmental stress and abrasion, compression and heat. In any case, belts of these various varieties can be made, for example, by impregnating a woven base fabric, which takes the form of an endless loop, with a synthetic polymeric resin. Preferably, the resin forms a coating of some predetermined thickness on the inner surface of the belt, so that the yarns from which the base fabric is woven may be protected from direct contact with the arcuate pressure shoe component of the long nip press.
It is typically this coating, which usually has a smooth, impervious surface to slide readily over the lubricated shoe and to prevent any of the lubricating oil from penetrating the structure of the belt to contaminate the press fabric, or fabrics, and fibrous web.
Furthermore, the opposite surface or outer surface is also coated. This surface can be smooth or can have voids, such as grooves or blind-drilled holes to receive water pressed from the paper web or press fabric(s).
Such a coating, for example, a urethane coating applied to a process belt (which may be either grooved or un-grooved), may also serve as a barrier material to prevent permeation of water from the paper side of the belt to the shoe side, where the urethane coating is constantly in contact with warm (˜50-60° C.) hydraulic oil.
In practice, during the operation of the long nip press, the belt is subjected to considerable mechanical and thermal stress. As the belt takes the form of an endless loop, it is directed through the long press nip subjecting the coating to a repeated stress that may ultimately lead to cracking of the coating.
Flex fatigue and cracking of the urethane coating of process belts is one of the shortcomings of current urethane material. This problem could be mitigated or eliminated by using softer or a less cross-linked urethane. However, softer (on an acceptable hardness scale like Shore C) or less cross-linked material tends to be less wear resistant and can allow groove closure in belts having grooves, which in turn reduces dewatering performance of the belts. Flex fatigue and wear are also problems with roll coverings used in paper machines.
Thus, there is a need to improve resistance to flex fatigue, crack propagation and wear, as well as delamination of urethane coatings in process belts and roll coverings, in addition to retarding permeation of water and oil; and in grooved belts resistance to groove closure.
For example, resistance to groove closure of the coating in a grooved belt typically needs resins of high dynamic modulus in the low strain regime; that is, strains of less than ten percent. In this connection, cast polyurethane elastomers are all segmented copolymers consisting of phases called “hard phase” and “soft phase.” In addition, these cast polyurethane elastomers may be made by a one-step process or a two-step process. In the one-step process, the macroglycol, isocyanate and curative (also called a “chain extender”) are all mixed together at one time. In the two-step process, the macroglycol and isocyanate are pre-reacted to form a prepolymer. This prepolymer is subsequently reacted with the curative. The latter approach is the most common one for making large castable parts.
Cast polyurethane articles include a wide range of forms and articles produced by pouring or pumping a reactive liquid polyurethane onto a substrate, or onto a mold. This broad category of polyurethane processing includes the single pass spiral (SPS) and multiple thin pass (MTP) coating processes that have been taught previously to produce process belts such as belts for shoe presses, shoe calenders and sheet transfer belts.
Increasing the dynamic modulus (of the polyurethane resin) typically requires increasing the volume fraction of the hard phase. This increase of the volume fraction of the hard phase can be achieved by increasing the weight percent of the isocyanate group (NCO), changing the type of NCO, or changing the composition of the curative.
However, increasing the modulus in this way generally increases the dynamic modulus as well as the breadth and location of the glass transition temperature. Therefore, in high strain-rate applications, such as papermaking process belt applications, the change in the weight percent of the hard segment content increases the risk of flex-cracking.
The above-noted polyurethane modifications which either increase dynamic modulus without changing the glass transition temperature, or increase energy dissipation at the crack tip, may in either case increase abrasion resistance of polyurethane coated process belts.
Heretofore, the use of nanoparticles to improve the barrier properties and other characteristics of coatings has been proposed.
U.S. Pat. No. 6,616,814 refers to the use of nanoparticles in a press belt. However, only the surface of an outer layer is equipped with the nanoparticles for wear purposes. It is said that the nanoparticles in the wear resistant outer surface(s) can be equipped with fluorocarbon chains to give the outer layer a hydrophobic characteristic.
U.S. Pat. No. 5,387,172 teaches fiber-reinforced plastic rolls coated with a synthetic resin and an abrasive filler powder (see, e.g. col. 3, lines 37-65) having various grain sizes (col. 3, line 66-col. 4, line 19).
U.S. Pat. No. 5,298,124 is a coated transfer belt for use in paper manufacturing. The coating is a type of polymer and may contain a kaolin clay particulate filler. This filler provides a surface roughness, which decreases with an increase in applied pressure.
U.S. Pat. No. 6,036,819 is a method for improving the cleanability of coated belts. The polymer coating may include a particulate filler similar to that disclosed in U.S. Pat. No. 5,298,124.
U.S. Pat. No. 6,136,151 is a press belt, press roll cover, or long nip shoe belt, which use a clay filler in the polymeric coating. It is an alternative to belts as taught in U.S. Pat. No. 5,298,124.
U.S. Pat. No. 4,002,791 is a woven fabric polyurethane-coated belt. The coating contains walnut shell powder to increase its coefficient of friction.
U.S. Pat. No. 4,466,164 is a supercalendering apparatus using an elastic roll. The core metal roll has a first coating of fibrous material with inorganic (quartz) filler loaded epoxy resin impregnated in the fibrous material and a second coating of inorganic filler loaded epoxy resin formed on the first coating.
U.S. Pat. No. 6,200,248 is a ceramic roll with coating compositions including mixtures of chromium oxide and titanium dioxide as well as aluminum oxide and zirconium oxide.
U.S. Pat. No. 6,200,915 is a lightweight textile fabric used for automobile air bags. Among other fillers, vermiculite and mica are used to lower the friction value.
U.S. Pat. No. 6,290,815 is a paper sheet or laminate containing grit particles, which give it a high abrasion resistance while retaining a glossy surface.
U.S. Pat. No. 6,331,231 provides a paper web transfer belt with good paper releasability. Closed bubbles, microcapsules, or a particulate filler are mixed into the polymeric resin coating.
The present invention is an alternative to those disclosed in the above patents for improving any and all of the abovementioned characteristics of urethane coated process belts and roll coverings.